Electroluminescent circuit element



United States Patent O atrasos ELECTRLUMBTESCENT CRCUH ELEMENT Edwin R. Bowerrnan, Jr., Whitestone, NX., assigner to General Teiephone and Electronics Laboratories, Enc., a corporation of Deiaware Filed Apr. 23, 196i, Ser. No. 166,344 8 Ciaims. (Si. 259-213) This invention relates to -circuit elements and devices utilizing electroluminescent and photoconductive materials.

Layers of electroluminescent :and photoconductive materials interposed between two electrodes have been employed in many types `of electronic circuit elements. In `designing these elements, an eort is usually made to reduce the distributed cap-acitances inherent in this type of construction and to obtain circuit elements in which the capacitances and resistances a-re essentially lumped. However, i have invented a new circuit element employing photoco-nductive and electrcluminescent layers in which the distributed capacitances `are used to provide novel electronic functions not heretofore obtainable.

Accordingly i-t is an object of my invention to provide a new type of circuit element employing electroluminescent and photoconductive layers.

lt is another object `of my invention to provide `a circuit element employing electroluminescent and photoconductive layers which has dis-tributed opticaland electrical characteristics.

Still another object is to provide a circuit element in which the capacitance increases with increases in applied voltage.

Yet another object is Ito provide la circuit element which gives a visual indication of changes in the magnitude of the applied voltage.

A fur-ther object is to provide a 'circuit element in which the current varies in a non-linear manner with changes in the magnitude of the applied voltage.

Yet a further object of the invention is to provide a circuit element employing electroluminescent and photoconductive llayers which may be used as a bistable device.

In the present invention a circuit element is provided in which a iirst electrode is atiixed to one surface of an electroluminescent layer. A photoconductive layer is yaiiixed to the other surface of the electroluminescent layer `and is opticly `and capacitively coupled thereto, the optical and capacitive coupling between the electroluminescent `and photoconductive 'layers being effectively distributed along the boundary between the two layers. A second electrode conductively connects one end of the photoconductive layer to a portion of the elec-troluminescent layer.

When a given voltage is lapplied across the iirst and second electrodes, the portion of the electroluminescen-t layer therebetween emits light. As the applied voltage is increased in magnitude, the light emitting area of the electroluminescent layer increases and illuminates more of the photoconductive layer. When light falls on the photoconductive layer its resistance decreases. Thus, increasing the applied voltage causes the illuminated area of the photoconductive layer to increase. Also, decreasing the resistance 'of the pho-toconductive layer causes the distributed capacitance between the electroluminescent and photoconductive layers to present a higher eective capacitance between the first and second electrodes as the magnitude of the applied voltage is increased.

In another embodiment of the invention, one surface of an electroluminescent layer is aixed to first and third spaced, transparent electrodes secured to a glass substrate. Second and fourth spaced transparent electrodes are atiiXed to the other surface of the electroluminescent layer, the distance between the second and fourth electrodes being several times greater than the distance between the first and third electrodes. A photoconductive laye-r is interposed between the second and fourth electrodes and is optically coupled to the electroluminescent layer. The ends of the photoconduotive layer are conductively connected to the second and fourth electrodes respectively while the portion of the photoconductive llayer between the second and fourth electrodes is in intimate contact with the electrolumine-scent layer.

By connecting the first and third electrodes together and coupling an alternating voltage source across the iirst and second electrodes, a device having `an input capacitance which increases with increases in applied voltage is obtained. Also, when the device is viewed through the iirst and third transparent electrodes, a bar of light appears on the electroluminescent layer which widens as the voltage is increased. If the voltage is raised suiiiciently, the entire photoconductor between the second and fourth electrodes is illuminated by the electroluminescent layer and becomes conductive.

In another application of the invention, an alternating voltage is coupled across the second and fourth electrodes. With this connection, the current iiowing 'between these electrodes is a non-linear function of the voltage across them.

A bistable device may be constructed using my invention by connecting a low impedance pulse voltage source between the first and third electrodes and an alternating yvoltage source between the rst and second electrodes, rPhe `area of the second electrode is made relatively small with respect to that Iof the fourth electrode and the gap between the first and third electrodes is closer to the second electrode than to the fourth electrode. When a short duration, high voltage pulse is applied between the first and third electro-des, the e-lectroluminescent layer is momentarily brightened illuminating the entire photoconductor between the second and fourth electrodes. The resistance of the photooonductor is reduced to a 'low value `and acts 4as the top electrode of the eleotroluminescent layer. As a result the entire electroluminescent layer remains lit until the alternating voltage source is removed.

The rabove objects of and the brief introduction to the present invention will be more lfully understood and fur-ther objects and advantages will become apparent from a study ort' the following description in connection with the drawings, wherein:

FG. 1 is a cross-sectional view of one fonm of my circuit element;

FIG. 2a is a plan view and FIG. 2b is a cross-sectional elevation view of another 'form of my circuit element;

FIG. 3a shows the circuit element connected to provide a device in which the input capacitance increases with increasing `applied voltage, and FIG. 3b is a graph showing the relationship between the applied voltage and input capacitance for the device of FIG. 3a;

FiG. 4a depicts the circuit element of FIGS. 2a and 2b connected to provide a device in which `the input circuit varies nonalinearly with changes in applied voltage, and FIG. 4b is a graph showing the relationship between the applied voltage :and the input current for the -device of FiG. 4a; and,

FIG. 5 is `an equivalent circuit showing the circuit element of FIGS. 2a and 2b connected to provide a bistable device.

Referring to FIG. 1, there is shown a cir-cuit element comprising a 4first transparent electrode A secured to a glass substrate ld. An electrolurninescent layer 11 (which emits light when a voltage is applied across it) is affixed to the ytop surface of electrode A. Electrode A may consist of any transparent, conductive coating, such as tin oxide, while electroluminescent layer il is formed of zinc sulfide activated with lcopper and coactivated with a chloride. A photoconductive layer i2, consisting of cadmium suliide or cadmium selcnide, is placed in intimate contact with the top Surface of the electroluminescent layer M. Photoconductive iayer i2 has a high electrical resistance when it is in the darli and a `considerably lower resistance when it is illuminated. A second electrode B conductively joins one end of the photoconductive layer 12 to a portion of electroluminescent layer 1.1.

When Ia voltage is applied between electrodes A and B, the portion of the electroluminescent layer 1l between the electrodes emits light` Iincreasing the voltage between electrodes A and B causes the light to fall upon photoconductive layer i2 increasing the area of the electroluminescent layer which is energized and increasing the capacitance of the device between the two electrodes.

Another embodiment of the invention is shown in the plan and elevation views of FGS. 2a and 2b respectively. In this form of the invention, irst and third spaced transparent electrodes A and C are secured to the glass substrate l5. An eiectroluminescent layer 16 is aixed to electrodes A and C filling Ithe gap between them. Second and fourth electrodes B and D respectively are secured to the top surface of electroluminescent layer le and a photoconductive layer `lil' is `deposited between the electrodes on the top surface of electroluminescent layer 15. Electrodes B and D maire contacts with the ends of photoconductive layer 17 while Ithe portion of layer i7 between the contacts maires intimate 4contact with electroluminescent layer i6.

Electroluminescent layer i6 by both capacitively and opticaliy coupled to photoconductive layer 17. The capacitive coupling is in the form of a distributed capacitance between the bottom electrodes A, C and the photoconductive layer 17; that is, photoconductive layer 17 acts effectively as `top electrode for electroluminescent layer 16. In the form of the invention shown in FIGS. 2a and 2b the gap between electrodes A and C is midway between electrodes B and D. As will be seen hereinafter, for some applications, the gap between electrodes A and C is not symmetrically located with respect to electrodes B and D.

In FIG. 3a there is shown an equivalent circuit of the device of FIGS. 2a and 2b. The photocondnctor layer 17 between electrodes B and D is represented by a variable resistor :Ja connected to the electrodes at each end. The distributed capacitance between electrodes A, C and :the photoconductive layer d? is indicated by the extension of electrodes A and B parallel to the photoconductor 17a. The space between electrodes A, B and photoconductor 17a. contains the electroluminescent layer 16a., the light emitted by the electroluminescent layer being indicated by the dashed arrows.

In the circuit shown in FG. 3a, an alternating voltage source 2S is coupled across electrodes A and B. Electrodes A and C are connected together. The applied voltage causes the portion of the electrolurninescent layer immediately between electrode B and electrode A to emit light. As voltage source is increased in magnitude, the light emitted fby eiectroluminescent layer 16a spreads over a greater area and impinges upon the photoconductor 17u thereby decreasing the resistance of a portion of the photoconductor near electrode B. Since the series resistance (that is the resistance of the photoconductor 17a.) is decreased, the capacitance between electrodes A and B increases as the distributed capacitance between the electrodes becomes more effective. Further increases in applied volta-ge causes further increases in capacitance producing the reiationship shown in FiG. 3b. The curve oi FIG. 3b was obtained with a circuit element having a gap between electrodes A and C of .005 inch at a frequency of 100G cycles per second.

When the device of FIG. 3a is viewed through transparent electrodes A and C, it is found that increasing the magnitude or" voltage source 2S increases `the illuminated area of electroluminescent layer 16a. The illuminated area begins at electrode B and spreads across the device as the voltage is increased ulti-mately reaching electrode D. Thus, the circuit element shown in FIG. 3a can be used both as a variabie capacitance device in which the capacitance increases with applied voltage and as a device which indicates the magnitude of an applied voltage.

FiG. 4a illustrates a form of the invention in which the voltage source is connected between electrodes B and D. The applied voltage produces an electric ield extending through electroluminescent layer 16a causing it to emit light. As the magnitude of voltage source 25 is increased, the brightness of the iight emitted by electroiuminescent layer lea. increases and the resistance of the illuminated portion of photoconductor 17u decreases. As a result, the current iiowing between electrodes B and D increases as a non-linear function of the applied voltage. The relationship between the current and applied voltage for a device having the same dimension as that shown in FIG. 3a is depicted in FIG. 4b.

In FiG. 5, the circuit element has been connected to provide a bistable device. A pulse source it? is connected through a transformer 41 across electrodes A and C and an alternating voltage source 25 is coupled between electrodes A and B. The area of electrode B is .made relatively small with respect to that of electrode D and the gap be'- tween electrodes A and C is closer to electrode B than to electrode D. When a short duration, high voltage pulse is applied by source 46 to electrodes A and C, the electroluminescent layer 16u lis momentarily brightened illuminating the entire photoconductor 17a. The resistance ot the entire photoconductor a. is thereby reduced to a low value and the photoconductor functions as the top electrode of the electroluminescent 4layer 16a. Under these conditions, the electroluminescent layer i will continue to emit light after the pulse source 40 has been energized. The device is returned to its initial state by disconnecting vol-tage source 25.

As many `changes could be made in the above construction and many diie-rent embodiments could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electroluminescent circuit element comprising an electroluminescent layer having first and second surfaces; a rst pair of spaced electrodes affixed to one surface of said electroluminescent layer; a second pair of spaced electrodes aiiixed to the other surface of said electroluminescent layer; and a photoconductive layer interposed between said second pair of spaced electrodes, said photoconductive layer being in intimate contact with said electroluminescent layer and optically coupled thereto.

2. An electroluminescent circuit element comprising an electroluminescent layer having first and second surfaces; a iirst pair of spaced electrodes affixed to one surtace ot said electroluminescent layer; a second pair of spaced electrodes symmetrically disposed with respect to said rst pair of electrodes atiixed to the other surface of said electroluminescent layer, the distance between said second pair of electrodes being greater than the distance between smid first pair of electrodes; and a photoconductive layer interposed between said second pair of electrodes, said photoconductive layer being in intimate contact with said electroluminescent layer and optically coupied thereto.

3. An eiectroluminescent circuit element comprising an electroluminescent layer having rst and second surfaces; first and third transparent electrodes aiiixed to one surface of said electroluminescent layer, said first and third transparent ciectrodes having adjacent parallel spaced edges; second and fourth spaced electrodes aiixed to the other surface of said clectrolurninescent layer, said second and fourth electrodes having adjacent edges extending parallel to said equidistantly spaced from the adjacent edges of said first and third electrodes; and a photoconductive layer interposed between said first and third electrodes, said photoconductive layer being in intimate Contact with said electrolurninescent layer and optically coupled thereto.

4. An electroluminescent circuit element comprising an electrolurninescent layer having first and second surfaces; first and third transparent electrodes affixed to one surface of said electrolutninescent layer, said first and third transparent electrodes having adjacent parallel spaced edges; second and fourth spaced electrodes afiixed to the other surface of said electroluminescent layer, said second and fourth electrodes having adjacent edges extending parallel to and equidistantly spaced from the adjacent edges of said first and third electrodes; a photoconductive layer interposed between said first and third electrodes, said photoconductive layer being in intimate Contact with said electrolurninescent layer and optically coupled thereto; means conductively connecting said first and third electrodes; and an alternating voltage source coupled between said first and second electrodes, the capacitance between said first and second electrodes increasing as the magnitude of said Voltage source is increased.

5. An electroluminescent circuit element comprising an electrolurninescent layer having first and second surfaces; first and third transparent electrodes afiixed to one surface of said electrolurninescent layer, said first and third transparent electrodes having adjacent parallel spaced edges; second and fourth spaced electrodes amxed to the other surface of said electrolurninescent la 'er, said second and fourth electrodes having adjacent edges extending parallel to and equidistantly spaced from the adjacent edges of said first and third electrodes; a photoconductive layer interposed between said second and fourth electrodes, said photoconductive layer being in intimate Contact with said electrolurninescent layer and optically coupled thereto; and an alternating voltage source coupled between said second and fourth electrodes, the current flowing between said second and fourth electrodes increasing non-linearly with increases in the magnitude of said voltage source.

6. A bistable electroluminescent device comprising an electrolurninescent layer having first and second surfaces; first and third transparent spaced electrodes affixed to one surface of said electroluminescent layer, said first and third electrodes having adjacent parallel spaced edges with a gap therebetween; second and fourth spaced electrodes having adjacent edges parallel to the adjacent edges of said first and third electrodes; said second electrode having a smaller area and being closer to the gap between said first and third electrodes than said fourth electrode, the gap between said second and fourth electrodes being greater than the gap between said first and third electrodes, a photoconductive layer interposed between said second and fourth electrodes, said photoconductive layer being in intimate contact with said electroluminescent layer and optically coupled thereto, an alternating voltage source coupled between said first and second electrodes, and a pulse voltage source coupled between said first and third electrodes, a voltage pulse applied from said pulse voltage source causing said electroluminescent layer to emit light until said alternating voltage source is removed.

7. An electrolurninescent circuit element comprising an electroluininescent layer having first and second surfaces; a photoconductive layer optically and capacitively coupled to said electroluniinescent layer, said optical and capacitive coupling being distributed along the boundary between said photoconductive and electroluminescent layers; first and third spaced electrodes affixed to the first surface of said electrolurninescent layer; and second and fourth spaced electrodes conductively joining the ends of said photoconductive layer to a portion of the second surface of said electroluminescent layer.

S. An electrolurninescent circuit element comprising a transparent non-conductive substrate; an electroluminescent layer having first and second surfaces; first and third transparent electrodes interposed between said substrate and the first surface of said electroluminescent layer, said rst and third transparent electrodes having adjacent spaced edges; second and fourth transparent electrodes afiixed to the other surface of said electrolurninescent layer, said second and fourth transparent electrodes having adjacent spaced edges extending in the same direction as the adjacent edges of said first and third electrodes, the distance between said first and third electrodes being small relative to the distance between said second and fourth electrodes; and a photoconductive layer interposed between said second and fourth electrodes, said photoconductive layer being in intimate contact with said electroluniinescent layer and optically coupled thereto.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ELECTROLUMINESCENT CIRCUIT ELEMENT COMPRISING AN ELECTROLUMINESCENT LAYER HAVING FIRST AND SECOND SURFACES; A FIRST PAIR OF SPACED ELECTRODES AFFIXED TO ONE SURFACE OF SIAD ELECTROLUMINESCENT LAYER; A SECOND PAIR OF SPACED ELECTRODES AFFIXED TO THE OTHER SURFACE OF SAID ELECTROLUMINESCENT LAYER; AND A PHOTOCONDUCTIVE LAYER INTERPOSED BETWEEN SAID SECOND PAIR OF SPACED ELECTRODES, SAID PHOTOCONDUCTIVE LAYER BEING IN INTIMATE CONTACT WITH SAID ELECTROLUMINESCENT LAYER AND OPTICALLY COUPLED THERETO. 